Thermal structure beneath Far Eastern Russia inferred from geothermobarometric analyses of mantle xenoliths: Direct evidence for high geothermal gradient in backarc lithosphere

Based on pressure and temperature (P-T) information of mantle-derived xenoliths, this report describes a geotherm in Far Eastern Russia, which is situated in the backarc of volcanic chains in eastern Asia. The mantle xenoliths have abundant CO2 fluid inclusions. Accurate determination of the internal pressure of the CO2 fluid inclusions enables estimation of the depth provenance of the mantle xenoliths. The depth provenances obtained from five mantle xenoliths are correlated with equilibrium temperatures estimated using a geothermometer. Assuming that the correlation reflects the geotherm in this region, it corresponds to heat flow of around 100 mW/m2, which is comparable to the surface heat flow in the volcanic arc. Such high heat flow implies a thin lithosphere and high temperature of the shallower upper mantle.Backarc areas are universally characterized by both high surface heat flow and the occurrence of recent volcanic activities. Furthermore low seismic velocity anomaly is widely observed in the shallower upper mantle in the backarc, especially in eastern Asia. The present xenolith geotherm suggests that the seismic anomaly arises from high temperature of the shallower upper mantle. It would be direct evidence for warming of the lithospheric mantle and subsequent lithospheric thinning, which is likely attributable to asthenospheric upwelling.

► We applied a geothermobarometry to mantle xenoliths from Far Eastern Russia. ► The P-T information of the mantle xenoliths corresponds to a heat flow of 100 mW/m2. ► The backarc is universally characterized by low-seismic velocity anomaly. ► The present high heat flow accounts for the cause of the seismic anomaly.